(https://i.imgur.com/7j7smaW.gif)
Falling lemmings can avoid being affected by objects if all the following conditions are met:
1) The object have to be very thin: specifically 2pixels or less
- I'm quite sure this has something to do with the fact that a normal lemming falls for 3pixels every frame.
2) The lemming have to start falling 3n+2 pixels above the object
3) The most weird thing: The object must have level space of 258 or more pixels above it
attached the level file ;)
Did some testing. Confirmed I can reproduce this using your test level. I also found that a Reacher can exhibit a similar behavior, also with the "must be low enough in the level" requirement.
Found it. A position check during the check-trigger-areas-on-intermediate-pixels code, was written in a hackish way that produced a side effect of comparing values modulo 256 instead of comparing them properly. Fixed in commit afb61e1.